Optical waveguides includes glass made ones and plastic made ones. Among them, the plastic optical waveguide is preferably used as a waveguide for illumination since the material itself is highly elastic and flexible.
An optical sensor transmits light and/or images and is composed of an optical waveguide as well as an optical image fiber and optionally a tube member and/or a continuous bore which transmit a fluid. FIG. 1 shows a cross section of one example of an optical waveguide for illumination to be used as a medical fiber scope. The optical waveguide 1 comprises a core 2 and outer and inner claddings 3 and 3'. The bores 4 surrounded with the inner claddings 3' are used for transporting a fluid or inserting an optical image fiber therein. The sizes of the core and cladding vary with the end use of the waveguide. Generally, a diameter of the core is from 0.25 to 1.00 mm, and a wall thickness of the cladding is from 10 to 20 .mu.m (0.01 to 0.02 mm).
The conventional plastic optical fiber predominantly comprises a core made of polymethyl methacrylate (hereinafter referred to as "PMMA") although it may comprise a core made of other transparent plastics such as polystyrene. However, the number of highly transparent plastics is not large. A commercially available plastic optical fiber comprises a core made of PMMA or polystyrene. Between them, the former is more important, since it has better optical transmission characteristics than the latter.
The polymethacrylate type optical fiber includes the following three types:
(A) An optical fiber comprising a core made of PMMA and a cladding made of a fluororesin.
This type of the PMMA optical fiber has good light transmission characteristics and low attenuation due to absorption and is widely commercially available. However, it has a drawback that it shrinks to a great extent at a temperature higher than 100.degree. C. For example, at 120.degree. C., it shrinks to a length of about 50% of the original length in several seconds. This is because the PMMA optical fiber is stretched during fabrication so as to impart flexibility to the fiber since the unstretched PMMA optical fiber has poor flexibility. Therefore, when heated, the stretched PMMA optical fiber recovers to or toward the original state.
(B) An optical fiber comprising a core made of PMMA containing 5 to 30% by weight of a plasticizer and a cladding made of a fluororesin.
Since this second type of the PMMA optical fiber is flexible due to the presence of the plasticizer in PMMA, it is not necessary to stretch the optical fiber during fabrication. Therefore, it shrinks only to a small extent. However, the cladding of this type of the optical fiber should be made thicker than that of the optical fiber of the type A since diffusion and migration of the plasticizer should be prevented by the cladding. For example, the thickness of the cladding is usually 100 to 500 .mu.m. Since light is transmitted through the core portion, the thicker cladding makes the core cross section smaller if the cross sectional area of the waveguide is the same so that the efficiency of light transmission of the waveguide becomes lower. In other words, for a constant cross section of the waveguide, it is preferable to make the core larger and the wall of the cladding thinner.
(C) An optical fiber comprising a core of polyisobutyl methacrylate and a cladding made of a fluororesin.
This optical fiber has substantially the same attenuation of light transmission and small shrinking rate as the PMMA optical fiber. However, this optical fiber is brittle and fragile, since polyisobutyl methacrylate is rigid and less flexible. Elongation at break is only about 5%. Thus, this fiber lacks the important advantage of the plastic optical fiber, namely resistance to bending and tension. The reason for this may be that isobutyl methacrylate has a branched butyl group.
It has been proposed to make an optical waveguide from a copolymer of isobutyl methacrylate/n-butyl methacrylate in a ratio of 4:1 to 2:3 (cf japanese patent publication No. 162849/1983), or PMMA or PMMA plasticized with adipate (cf. Japanese patent application No. 162847/1983). However, these optical waveguides are not completely satisfactory. For example, PMMA or the isobutyl methacrylate/n-butyl methacrylate copolymer is brittle and does not have enough flexibility. If the waveguide is stretched to impart flexibility to it, it shrinks when heated. PMMA plasticized with adipate has inferior light transmission characteristics since the plasticizer deteriorates the transparency of PMMA and scatters light. In addition, it is very difficult to obtain highly pure adipate by purification.
Hitherto, for the production of an optical waveguide containing an image fiber, there has been proposed several methods. Among them, the most advantageous methods are a method proposed in Japanese patent application No. 162847/1983 which comprises co-extruding a core material and a cladding material with simultaneously supplying a metal wire to a co-extrusion die to form an optical waveguide having the metal wire therein and then withdrawing the wire from the waveguide to form a bore therein. A method proposed in Japanese patent application No. 25866/1984 comprises co-extruding a core material and a cladding material with simultaneously supplying a hollow fiber made of a polymer or quartz to a co-extrusion die to form an optical waveguide having a bore therein.
However, the above proposed methods are not suitable for producing an optical sensor having a small outer diameter. This is because the optical image fiber or bundle should be inserted in a thin bore made in the optical waveguide during which the waveguide tends to be damaged by the inserted image fiber or bundle. In addition, by these methods, it is difficult to produce an optical sensor with flexibility and bending strength since the sensor contains the unstretchable metal wire or hollow fiber.